I got a Rodenstock 240mm F/5.6 S in great condition, and while mounting
it on a lensboard I became puzzled by the retaining ring and spacer. I
am not certain what the spacer is for. The front and rear elements
appear to join properly without the spacer. In fact, with the thick
board, the spacer leaves few threads to engage the nut.

I've found no manual (yet) for it.

Any clues?